DIA-HELIOTROPISM AND DIA-GEOTROl'ISM 65 I 



extent in leaves is in the parallel-veined leaves of monocoty- 

 ledons. This fact, that the lamina is not in general the per- 

 ceptive organ with regard to stimulus, say of light, is still 

 further demonstrated in the following experiment, performed 

 on the leaflet of Erythrina indica. Sunlight was first applied 

 locally to the lamina of this leaflet by means of a suitably 

 inclined mirror, at the point in the record which is marked x 

 (fig. 262). It will be noticed that during ten minutes of such 

 application not the slightest responsive movement was in- 

 duced. Keeping everything else the same, the light was 



Fig. 262. Record showini; thai Lamina is not the Perceptive Organ 



Light applied vertically at x on lamina t)f Erythrina indica, and con- 

 tinued for ten minutes, produces no response ; but when applied on 

 the puh inus at the arrow { \ ) there is immediate positive response. 

 Dotted line shows positive after-effect and recovery on cessation of 

 light. 



now shifted by a movement of the mirror, and thrown 

 directly on the pulvinus, at the point in the record which is 

 marked with an arrow (t). It will be observed that the leaf 

 began to respond immediately by a movement towards the 

 light, and in the course of seventeen minutes' exposure its 

 tip moved through a distance of 20 mm. or at an average 

 rate of a little over i mm. per minute. On the withdrawal 

 of light the movement persisted, owing to the positive after- 

 effect, for a period of seven minutes, after which recovery 

 began. The marked difference between the quiescent con- 



